We are currently involved in four lines of experimentation that deal with the biology and chemistry of nucleic acids. This work is made possible by the NIH grant GM 17088, and depends heavily upon the use of the electron microscope. 1. The first line of experimentation deals with the isolation and characterization of the DNA element which serves as the origin for phi X DNA synthesis. 2. A second line of experimentation involves the isolation and sequencing of the 3 DNA regions in bacteriophage phi X which function as initiation sites for messenger RNA synthesis. 3. The third set of experiments concerns the isolation of immunoglobulin mRNA with the objective of studying the arrangement, and possible rearrangement during development, of immune system DNA. Specifically, we wish to search for physical evidence to support the translocation hypothesis which is generally invoked in germ line theories of antibody diversity. 4. Our fourth line of experimentation deals with the electron microscopic observation of DNA molecules that are almost certainly intermediate in the process of genetic recombination.